Plants belonging to the genus Allium such as Welsh onions, common onions and garlics have long been consumed as foods having a tonic effect. Recently, it has also been known that an L-cysteine sulfoxide derivative, which is a sulfur amino acid contained in plants belonging to the genus Allium, has the enhancing action on the production of the male hormone (androgen), testosterone (Patent Literature 1).
However, it is difficult to ingest sulfur amino acids continuously in an amount enough to expect physiological actions only by eating plants belonging to the genus Allium. Also, because plants belonging to the genus Allium contain not only sulfur amino acids, but also various other components, eating plants belonging to the genus Allium themselves is not efficient as a means for ingesting sulfur amino acids. Therefore, there is a demand for the development of a method for concentrating the sulfur amino acid contained in plants belonging to the genus Allium and a composition comprising a high content of sulfur amino acid derived from plants belonging to the genus Allium. 
The aforementioned Patent Literature 1 describes a method for producing a processed product of a plant belonging to the genus Allium comprising a high content of an L-cysteine sulfoxide derivative, comprising deactivating C-S lyase, which degrades sulfur amino acids contained in a plant belonging to the genus Allium, by subjecting an uncut plant to a heat treatment for 5 to 120 minutes under the conditions of a pressure of 1 to 5 atm and a temperature of 40 to 150° C. Also, the Patent Literature 1 describes that an extract containing an L-cysteine sulfoxide derivative from the aforementioned processed product of a plant belonging to the genus Allium is obtained by means of alcohol extraction and concentration under reduced pressure. Also, Patent Literature 2 describes a method for producing a high concentration of S-alkenyl cysteine sulfoxide by treating the edible part of a plant belonging to the family Liliaceae with mannanase, cellulase and pectinase, all of which are plant cell wall-degrading enzymes, and then subjecting the resulting product to an ion exchange resin treatment.
However, all of the products obtained by the aforementioned methods have not been fully satisfactory in terms of the sulfur amino acid content.